The final assembly of semiconductive devices or chips to form electronic components is uniquely distinct from preceding wafer processing steps in that in the final assembly, operations are typically performed on individual units rather than simultaneously on a large number of units in a batch processing mode. Thus, in final assembly operations, or packaging operations, as they are often referred to, a large number of repetitive and mostly mechanical assembly operations can easily degrade in quality to render a significant portion of the end product, the electronic components, defective. A continuous quality surveillance effort appears to be in order.
Inspection of, for example, electronic components such as semiconductor packages, or any of their subassemblies, is encumbered by their small size with respect to what is typically perceived by the human eye. Microscopes and microscopic images on video screens are typically employed in the inspection of electronic devices or small electrical devices in general to aid the human operator.
A particular failure mode in small electrical device assemblies relates to bonded interconnections between the devices themselves and a lead, or leads, which couple terminals of such devices to external connections. Often an interconnecting contact between a lead and a device is established electrically, but is of insufficient strength to endure a plastic molding operation or to remain intact after temperature cycling. Thus, a device may pass an electrical test prior to plastic encapsulation, but fail electrically after being plastic encapsulated, where the plastic flow in the mold forming the final housing or package has pulled away a lead from the device. The resulting damage may be even more aggravating when the separation of a lead from the device is only partial, so that an electrical failure of the final component is not immediately detected or does not occur until the device is installed in a large piece of apparatus which has been placed into operation.
Testing of the bond strength of leads to small devices prior to such devices becoming packaged, either by plastic encapsulation, or by other means, is therefore particularly important when the manufacture of components of high integrity is desired. However, even though such testing occurs on articles of comparatively very small size, strength test costs are desirably minimized, hence any testing is preferably performed quickly to minimize operator time.
Existing commercial lead test apparatus employs a pedestal for mounting a device the leads of which are to be tested. A vertically movable arm which is positioned above the pedestal, supports a clamp. The clamp is functionally attached to a typical transducive weight sensor of the type commonly referred to as a load cell. When the apparatus is operated, a device to be tested is mounted to the pedestal, and a lead, the bond strength of which is to be tested is aligned with the clamp. The clamp is then closed to grasp the lead to be tested. The vertical movement of the arm is thereafter activated to gradually move it away from the device with an ever increasing pulling force on the lead. During this time, the load cell on the arm registers an increasing force which appears on a digital readout of the cell. The test operator observes and records the highest obtained reading on the lead prior to its breakage or separation from the device.
Problems with such test apparatus have been experienced in that in closing the clamp the leads tend to become damaged to cause at times a lead to break before the strength of the bond of the lead to the device is determined. More importantly, however, the testing cycle to test a single lead including repositioning the device to align a lead with the clamp may require between three and four minutes, such that approximately an hour's time is required to test all leads of a typical 16-leaded device.
It is, therefore, desirable to improve available lead strength testing methods and apparatus to furnish current data from statistical samples of tested devices to operators controlling the final assembly of the corresponding devices.